This application relates generally to transmit active phased array antennas with multiple shaped beams. More specifically, this application relates to suppression of intermodulation-product peak value radiated from active phased array antennas over a coverage area.
Active phased array antennas are generally suitable for applications in which multiple high-directivity beams are to be generated with large signal bandwidth, as may be appropriate for broadband applications such as multimedia and video conferencing. The suitability of active phased array antennas for such applications derives from the fact that they may be configured in orbit to provide bandwidth on demand anywhere within an identified service area, and thereby maximize the overall system utilization. Furthermore, active phased array antennas with phase shifters enable a variety of different types of beam characteristics. For example, such antennas may support configurations that correspond to one or a few channels per beam, shaped beams, scanned beams, and beams that are reconfigurable with respect to shape, beam direction, frequency, and power.
A particular challenge for these types of systems is to optimize the payload capacity under the constraint of meeting certain performance requirements. Because of cost and size considerations, transmit active phased array antennas typically use solid-state power amplifiers (xe2x80x9cSSPAxe2x80x9d), which are operated in a linear region at wide bandwidth to accommodate the fact that each element amplifier may see all carriers in a signal. Such operational parameters result in a low power-conversion efficiency, i.e. of the order of 20%. Accordingly, the high power subsystem weight and thermal-dissipation needs of such SSPAs tend to drive the cost of the payload. Increases in efficiency of the use of SSPAs in active phased array antennas may thus have a significant impact on the overall cost of a spacecraft that carries the antenna.
A particular limiting factor in the efficiency of SSPAs in active phased array antennas is the power associated with production of intermodulation products. As an SSPA is driven into a more efficient nonlinear region, power is transferred from the carriers into intermodulation products, and those intermodulation products that fall in the carrier bands degrade the signal-to-noise level. One approach that is sometimes taken to reduce the strength of intermodulation products is to back off the power of the SSPAs by about 2-3 dB, although such an approach further compromises the overall efficiency of the SSPAs.
There is accordingly a general need in the art for suppressing intermodulation products for active phased array antennas.
Embodiments of the invention thus provide a method for determining phase distributions for use in transmitting signals with an active phased array antenna having a plurality of beams and a plurality of carriers. Distinct initial aperture phase distributions corresponding to the plurality of carriers are allocated. The aperture phase distributions are optimized to simultaneously increase carrier-signal power and reduce an intermodulation product radiated in the respective coverage areas in accordance with amplification and radiation of signals having the modified aperture phase distributions. In some embodiments, the initial aperture phase distributions are modified for each of the plurality of carriers to generate respective radiation patterns that substantially correspond to respective coverage areas in accordance with amplification and radiation of signals having the initial aperture phase distributions.
The methods of the invention may support different beam configurations. For example, in some embodiments, the respective coverage areas for the plurality of carriers is substantially the same. Also, embodiments of the invention may make use of different initial aperture phase distributions. In one embodiment, at least one of the initial aperture phase distributions is substantially paraboloidal. In another embodiment, at least one of the initial aperture phase distributions is substantially hyperbolically paraboloidal. A first of the initial aperture phase distributions may have two or more orthogonal planes of symmetry about an axis orthogonal to a first aperture plane and a second of the initial aperture phase distributions may be asymmetric about the axis. In some embodiments, some of the initial aperture phase distributions may advantageously be transforms of each other. For example, in one embodiment, a second of the initial aperture phase distributions is substantially equal to a first of the initial aperture phase distributions subject to complex conjugation and a 180xc2x0 rotation in an aperture plane.
In various embodiments, the intermodulation product may comprise a third-order intermodulation product, may comprise a fifth-order intermodulation product, and may comprise an in-band intermodulation product.
In some instances, the active phased array antenna may further comprise a second plurality of carriers, with second initial aperture phase distributions corresponding to the second plurality of carriers also allocated. Each such second initial aperture phase distribution is substantially equal to one of the initial aperture phase distributions. The second initial aperture phase distributions are modified for each of the second plurality of carriers to generate respective radiation patterns that substantially correspond to respective coverage areas in accordance with amplification and transmission of signals having the second initial aperture phase distributions. The modified second aperture phase distributions are optimized to simultaneously increase carrier-signal power and reduce an intermodulation product radiated in the respective coverage areas in accordance with amplification and transmission of signals with the modified aperture phase distributions and second modified aperture phase distributions. In one such embodiment, each of the plurality of carriers and the second plurality of carriers is less than five in number.
Embodiments of the invention also include methods for transmitting a plurality of shaped beams with an active phased array antenna having a plurality of carriers. A first of the plurality of shaped beams is transmitted with a first of the plurality of carriers and a second of the plurality of shaped beams is transmitted with a second of the plurality of carriers. The plurality of shaped beams have aperture phase distributions determined as described above.
The methods of the invention may also be embodied by an active phased array antenna. The active phased array antenna has a plurality of antenna elements, a plurality of filter elements coupled with the antenna elements, a plurality of amplifier elements coupled with the antenna elements, and a plurality of shaped beam ports. A beamformer is provided having a plurality of elemental paths for coupling the beam ports with the amplifier elements. The beamformer includes phase shifters adapted to provide aperture phase distributions to the amplifier elements in accordance with the embodiments described above.